Caution !
I used Future only once, as a clear coat on decals, and found that it dissolved when the decal was immersed in water.
As it's a wax floor polish, designed to be removed with water, this is not surprising, and I've never used it since.
However, a standard acrylic gloss varnish is fine, and I use a spray can, sometimes Humbrol, or an artist's varnish, my most recent being Vallejo clear gloss artist's acrylic, in an aerosol spray can. This is used on any decal sheet which is even slightly suspect, especially old decals. A single coat normally does the job, sometimes a second coat, sprayed at least 15 minutes after the first coat, is applied if the decals are really aged.
Leave the decal sheet for at least 30 minutes, preferably longer, before use.